When, for example, turbine exhaust gases are passed to a waste heat boiler through a duct it may be necessary to provide means for relieving excess pressures in the duct. Also if it is required to shut down the waste heat boiler for the purposes of, for example, maintenance, the exhaust gases from the turbine will have to be diverted. In order to do this gases can be prevented from reaching the boiler by closing a damper situated in the duct in front of the boiler. The gases can then be diverted through a branch duct leading to, for example, a stack. This branch duct will normally have a damper positioned in it which is opened and through which turbine gases can be passed during shut-down of the boiler. This damper could provide pressure relief when gases are being passed to the boiler.